This invention relates to cable guided intrusion detection systems and, in particular, to a system having an FM CW sensor using helically wound coaxial transmission lines to locate the intruder and using a location dependent threshold to declare the presence of an intruder.
Cable guided radar has been used to detect intruders since the early 1970""s. One of the earliest leaky coaxial cable intrusion sensors is the subject of U.S. Pat. No. 4,091,367. In this system, parallel leaky coaxial cables are buried around the perimeter of the site being protected. A pulse of RF energy is transmitted along one cable to setup an external electromagnetic field that propagates along the length of the cable. The second leaky coaxial cable receives energy reflected from the intruder thereby returning a portion of the transmitted pulse back to the receiver. The time delay between the onset of the transmit pulse and the receipt of the reflected pulse is used to determine the location of the intruder along the length of the cable pair. In order to compensate for attenuation, xe2x80x9cgraded cablesxe2x80x9d in which the aperture size increases with distance are used. While many different means of grading cables have been developed, all such techniques increase the cost of the cable.
In the system disclosed in U.S. Pat. No. 4,091,367, a 400-nanosecond pulse with a carrier frequency of 60 MHz is used. An analog to digital converter is used to find 84 In-phase and 84 Quadrature samples of the received signal from a 5280-foot long cable. This provides a digital sample for 62-foot cells or segments along the length of the cable pair. Based on a calibration walk, a separate threshold is applied to each cell. One factor limiting the performance of the system described in U.S. Pat. No. 4,091,367 is the relatively low duty cycle. The 400 nanoseconds pulse width and a 30 kHz repetition rate limits the duty cycle to about 1.2%. The FM CW approach utilized in the present invention allows for up to a 100% duty cycle and hence a significant improvement in Signal to Noise Ratio (SNR).
A second factor limiting the performance of the system described in U.S. Pat. No. 4,091,367 is the substantial variation in sensitivity within each 62-foot cell. Typical soil conditions and installation practice created up to 15-dB variation in the response within each 62 foot cell. One factor contributing to this variation is multipath field cancellation due to its relatively narrow bandwidth to carrier frequency ratio of 4.1%. A second factor is the multiple reflections on the two-wire line formed by the outer conductors of the separate transmit and receive cables. With only one threshold per cell, a threshold set to ensure 95% Probability of Detection (Pd) could detect small animals as nuisance alarms at the more sensitive locations within the cell. In the present invention, the location of the intrusion is determined within 1 to 2 meters prior to applying a threshold thereby overcoming this problem. In addition, the bandwidth of the FM CW chirp transmission and the elimination of the two-wire line mode obtained with this invention substantially reduces the sensitivity variation along the length of the transducer cable.
The high-speed logic associated with pulse cable guided radar described in U.S. Pat. No. 4,091,367 and the large diameter leaky coaxial cable that it uses result in a relatively costly perimeter security product. This lead to the development of CW sensors with distributed processing such as described in U.S. Pat. No. 4,562,428. This type of system reduces the cost of leaky cable perimeter security, however, it introduces several problems. Because there is no ability to locate the intruder along the length of the cable in a CW system, one threshold is applied to the entire length of cable. Typically, these cables are 100 to 150 meters long. To make this system operable, the cable has to be graded. A graded cable is one in which the apertures are increased with distance along the length of the cable to compensate for attenuation. Even with a perfectly graded cable there is increased variation in sensitivity along the cable length when compared with the 62-foot cells of the pulsed cable guided radar of U.S. Pat. No. 4,091,367. This is a source of nuisance alarms.
Leaky coaxial cables such as that described in U.S. Pat. Nos. 4,300,388, 4,599,121 and 4,660,007 or some of those illustrated in U.S. Pat. No. 4,091,367 have diamond shaped apertures that are comparable in size to the cable diameter and support both magnetic and electric field coupling. The electric field coupling (sometimes referred to as capacitive coupling) is affected by the dielectric constant of the medium surrounding the cable. This can lead to significant changes in the strength of the external electromagnetic fields when the cable is buried in wet soil as it freezes. Secondly, if mounted above ground these cables support external modes of propagation which cause large periodic variations in sensitivity. This mode cancellation problem has limited these cables to buried applications.
There are a number of cables with continuously slotted outer conductors such as that described in U.S. Pat. No. 5,834,688 wherein the continuous slot can be used for grading. The cable described therein has a second outer conductor made from conductive plastic with the conductivity of the plastic jacket selected so as to limit electric field coupling while accentuating magnetic coupling. This cable is made costly to produce by the grading of the foil outer conductor and the use of conductive plastic second outer conductor. The conductive plastic is expensive, difficult to work with and requires a separate extrusion process.
When installed with the cables buried in the ground, the cost of installation of these two cable systems is very significant. The introduction of the Siamese or twin leaky coaxial cable described in U.S. Pat. No. 5,247,270 was the first attempt at producing a single cable system so as to essentially reduce the installation cost by a factor of two. This cable has two metallic outer conductors. The first outer conductor is a continuously slotted foil designed to provide cable grading and the second is a helically wrapped steel wire to support magnetic field coupling while minimizing electric field coupling from inside to outside of the cable. This patent reference describes the virtues of magnetic coupling as opposed to electric field coupling. The continuous taper of the first foil outer conductor and the high pitch steel winding of the second outer conductor make this cable expensive to manufacture.
The present invention utilizes FM CW signal processing to locate intruders along the length of the cable while eliminating the need to grade the cable thereby making the single helically wrapped outer conductor cable described herein considerably lower in cost than existing leaky cable structures. The complete circumferential coverage of the outer conductor of the new cable ensures magnetic field coupling without electric field coupling. It also provides a slow wave structure to facilitate the use of the cable above ground as well as in buried applications.
U.S. Pat. No. 5,446,446 describes an acoustical cable perimeter security sensor employing a coded pulse transmission. While this sensor detects motion of the cable and does not have external electromagnetic fields, it does locate the intruder using an ultra-wideband transmission. This ability to locate has proven to be very beneficial in allowing the installer to create detection zones in software. The flexibility of this feature is very important when using the sensor with CCTV assessment. This xe2x80x9cFree Format Zoningxe2x80x9d benefit is attained using the FM CW cable guided radar system which is the subject of the present invention. Furthermore, the ability of the subject invention to locate the intruder before applying the threshold has proven to be very effective in overcoming variations in sensitivity along the length of the cable. This xe2x80x9cSensitivity Levelingxe2x80x9d benefit is provided in the subject FM CW cable guided radar.
In summary, the FM CW cable guided radar described herein provides a cost effective perimeter field disturbance sensor with a high duty cycle along with all of the benefits associated with the ability to locate an intruder along the length of the cable transducer and reduces the likelihood of indicating a false alarm condition by using location specific thresholds.
The present invention uses a chirp FM CW transmission on one leaky coaxial transmission line to create an external electromagnetic field, which is monitored, by a second leaky coaxial transmission line in a Siamese or twin cable construction to detect and locate intruders. By time sharing the FM CW signal processing circuitry between two cables, a 50% duty cycle is achieved which has the beneficial Signal to Noise characteristics of a CW sensor as well as the ability to locate the intruder along the length of the cable.
The ultra wide bandwidth of a HF band chirp transmission minimizes the variation in sensitivity along the length of the cable by averaging over the sweep. The present system utilizes a cable wherein the outer conductors of the transmit and receive coaxial lines are in continuous electrical contact along the length of the cable thereby eliminating any two-wire line mode between the outer conductors of the two coaxial lines. The helical outer conductors on the two coaxial lines are counter wound. The helical nature of the outer conductors is designed to support a surface wave and maximize magnetic field coupling while minimizing capacitive coupling. Magnetic coupling minimizes the environmental effects and the surface wave can be supported with the cables either buried or above ground.
Quadrature detection is used to generate complex inputs to a Fast Fourier Transform (FFT). Both the frequency and phase output of the FFT are used to accurately locate the intruder along the length of the cable. This location information is used to apply a location specific threshold to the response amplitude in order to compensate for the variations in sensitivity along the length of the cable. Each processor operates with two lengths of Siamese cable extending in opposite direction from the processor. An upward sweeping chirp is applied to one cable and a downward sweeping chirp is applied to the other so as to minimize interference between multiple sensors.
This system provides a higher probability of detection with a lower false alarm rate at a significantly lower cost than the systems described in the prior art. Further features and advantages of the invention will become more readily apparent from the following description of a preferred embodiment when taken in conjunction with the accompanying drawings.